Have you been using the Tamper Data extension in an earlier version of Firefox? Is the problem that there is no visible way to call up or configure Tamper Data? I don't use this extension myself, but generally speaking, you could check the following locations: (1) Tools menu To display the classic menu bar temporarily, tap the Alt key. To make it persistent, after tapping Alt, use View > Toolbars > Menu Bar (2) Developer Tools menu - in case it now integrates here * "3-bar" menu button > Developer * menu bar: Tools > Web Developer (3) Unused buttons - in case it has a button that just isn't on any toolbar Use the Customize feature to check the available buttons and drag to the desired location. See: [[Customize Firefox controls, buttons and toolbars]]. Can you find a way to run it?